1. Field of the Invention
The present invention relates to a method of making a rigid or semi-rigid lightweight foamed material which exhibits excellent stiffness, heat insulation, acoustic insulation and moisture resistance. The present invention further relates to a composite panel made from such a rigid or semi-rigid lightweight foamed material which can be used for forming a molded ceiling or a door panel for an automobile, a partition or screen for interior use, and the like. 2. Description of the Related Art
Conventionally, a rigid or semi-rigid plastic foam material is used to form interior finishes of automobiles such as molded ceilings, door panels and the like because such materials are lightweight and exhibit heat insulation and acoustic insulation properties.
Such plastic foams have been used as a core material for foaming a composite panel by laminating inorganic reinforcing sheets such as a glass fiber mat, a carbon fiber mat or a non-woven cloth made of polyester fiber or polyamide fiber on one or both sides through the use of adhesives. However, the foregoing conventional method of forming plastic foams has a number of problems. For example, although the inorganic reinforcing sheet such as the glass fiber mat, exhibits excellent reinforcing characteristics, forming a glass fiber mat pollutes the work environment and irritates the worker's skin. Furthermore, cutters used to trim the edges of such products are easily worn and damaged by such glass fiber mats. Even further, the glass fiber mat is relatively heavy so that the finished product is heavy.
Non-woven cloth made from polyester fibers, on the other hand, does not have sufficient strength to yield a desired reinforcing effect and anisotropic in tensile strength. Therefore, such materials cannot be practically used as a reinforcement. Furthermore, as the non-woven cloth has a large elongation rate, particularly at higher temperatures, the stiffness of a composite panel reinforced with non-woven cloth will decrease remarkably at higher temperatures.
In the case where severe follow-up property to a mold is required as in the case of deep draw molding, the lightweight foam or the core-material should be thermoplastic. However, such a core-material will lessen the heat resistance of the final product and a panel with sufficient stiffness at high temperatures cannot be produced.
If the rigid or semi-rigid foamed material is formed from polyurethane foam, phenol foam, and the like by the one shot method, it is generally produced as a block. When the block is sliced and a board thus foamed is applied to press working, it is apt to cause cracking or splitting which is especially disadvantageous for deep draw molding.
To solve the foregoing problem, it has been proposed that the liquid raw materials of the foam be cast into a mold to be foamed and cured in the mold. However, there are problems associated with the moldability such as insufficient fluidity of the foamed material to fill gaps of under 10 mm in width and a deterioration of the surface condition of the molded product. There is also the problem of increase in surface density of the molded product which causes an increase in the total weight of the product and a difficulty in manufacturing composite materials by such process.
It was recently proposed that an isocyanate component(B) be impregnated into a flexible porous material(A) and reacted with water to form a network structure to improve the moldability (Japanese Patent Publication Tokkosho 61-51544, USP 4451310, Japanese Publication Tokkosho 57-22013 and Japanese Patent Provisional Publication Tokkaisho 58-53465). However, this method may deteriorate the working environment as a result of the impregnated isocyanate component(B). Furthermore, when a soft polyurethane foam is used as the flexible porous material, it may swell as a result of the impregnation with isocyanate component. This in turn lowers the foam strength and results in a decrease in workability and dimensional stability of the material. As a result, the impregnated quantity of isocyanate component must be increased to make up for the strength of rigid foam material. This, in turn, disadvantageously increases the weight of the product.
The mechanism of molding and curing the flexible porous material and isocyanate component is not a chemical reaction but a formation of simple physical composite. As the flexibility necessary for molding depends upon the flexible porous material produced beforehand and the isocyanate component independently reacts with water to be cured, the final product is considered to be a physical composite of the flexible material and the cured material.